1. Field of the Invention
The present invention relates to pneumatic radial tires having a directional tread pattern and more particularly to such a pneumatic radial tire suitable for all seasons.
2. Prior Art
As shown in FIG. 3, there are known pneumatic radial tires having a directional tread pattern 30 oriented with respect to the direction (R) of rotation of the tire. This tread pattern 30 comprises four straight main grooves 31 extending in the circumferential direction (T) of the tire, a straight circumferential sub-groove 32 extending over an equator (CL) of the tire, and a plurality of lateral sub-grooves 33 extending at the same angle of inclination across the main grooves 31 outwardly from the circumferential sub-groove 32 in the direction of width of the tire, whereby the main grooves 31 together with the circumferential sub-groove 32 and the lateral sub-grooves 33 form a plurality of multi-directional blocks 34 in separate relation to one another.
Recently, there has arisen a strong demand for a tire having good wet and snowy road performance for use in all seasons. Generally, for high performance under snowy conditions, tires are required to have a value of at least 100 in snow traction index (STI) represented by projected area of a pattern edge with respect to the radial direction of the tire. However, while tires having the above mentioned tread pattern of FIG. 3 excel in draining characteristics, they are not satisfactory in on-snow performance such as braking performance and turning performance because the STI value is less than 100. Hence, the tires are not applicable for use in all seasons.